The direction finding systems of today owe much to concepts related to radio direction finding learned over the first half of this century. However, over the past two decades, relatively inexpensive analog and digital processing tools have lead to the development of high performance, easy to use, direction finding devices that are viable for many applications.
While these new tools are derived from advances in commercial digital signal processing (DSP), there are technologies unique to direction finding (DF) systems that are of significant importance to such systems. The design of a complete DF system requires careful consideration of many factors ranging from frequency, propagation, and modulation to application and deployment. However, the most important direction finding fundamental is the method used to intercept and locate signals of interest (SOI).
As with any radio frequency signal intercepting system, the receivers and detectors are optimized for the particular signals of interest. In such systems certain design performance requirements such as size, weight and power are balanced against the key systems specifications: selectivity and sensitivity. One of the most critical parameters to be influenced by DSP functions are the computation of the bearing angle of the signal. The present patent application is directed towards influencing the computation of bearing angles. DSP plays a major role in improvement of the signal to noise ratio and computation of the quality factors, an indication of the effectiveness of the bearing measurement. Processing also considers calibration issues. Specifically, various antenna and equipment calibration methods use DSP to achieve and maintain the desired measurement accuracies on the order of a few degrees of true compass headings.
Finally, the DF system processor provides the man-machine interface. With new digital display technology, many additional capabilities have been added to the design of today's direction finding systems.
The functional features of the system include intercept search speed and operational performance as well as adaptability of the equipment to multiple applications. There are many types of advanced processing procedures and functions associated with DSP systems. In addition, these performance characteristics improve the direction finding systems.
However, the performance of even the latest direction finding systems is ultimately affected by the law of physics. The chance location of the direction finding (DF) equipment at a field site or on a platform may result in significant measurement errors. In some cases the deleterious effects of the site or platform can be mitigated either by moving the location or calibrating the error into the calculation.
In antenna array sampling techniques, various methods are used to achieve affordable multi-application system designs. These sampling methods determine the bearing angle of the radio signal so that the proper signal can be found.
Antenna array sampling methods, like those used in current pseudo-Doppler DF, are an attractive way of achieving affordable multi-application system designs. When combined with the latest signal processing methods and low-cost processors, array sampling techniques offer benefits that are comparable to earlier, very expensive system approaches.
DF antennas have aperture dimensions that are typically small, most less than half a wavelength. Today's electromagnetic modeling and computer-aided design tools allow engineers to design antennas that are relatively small and efficient over wide spectral bands. But the small aperture size of the antennas would make turning or steering techniques an ineffective means to locate the arriving signal bearing. Instead, three fundamental measurement parameters are used; amplitude, phase and Doppler frequency.
In the amplitude measurements, two sets of antennas with dipole-type patterns are arranged orthogonally. Signals arriving at the two antennas induce a voltage relative to the polarization and radiation pattern for each antenna. The bearing angle is derived from the ratio of the two signal amplitudes. Because simple amplitude measurements are made, a sense antenna (with an omni-directional pattern) is used to resolve the "180 degree" ambiguity in the bearing calculation.
Direct phase measurement methods for example, use a set of four antennas. Two antennas form one baseline. The relative phase differences of the induced voltages between the antennas define the bearing angles. Ambiguity in the bearing measurements, therefore, is not an issue.
Conceptually, at least, the Doppler measurement methods are straightforward. An antenna is rotated about a point at a given angular rate. As the antenna moves, it imposes a Doppler shift on the arriving signal. The magnitude of the Doppler shift is at a maximum as the antenna moves directly toward and away from the direction of the incoming wavefront. There is no apparent frequency shift when the antenna moves orthogonally to the wavefront. The bearing angle is therefore proportional to the relative position of the zero crossings of the Doppler-shifted signal. For many applications Doppler methods are not practical, since the system uses motors and moving components.
Although the amplitude and phase measurement methods are viable DF approaches, they too have some significant constraints. First, they must maintain amplitude and phase balance to minimize measurement errors. Maintaining amplitude and phase balance is especially challenging when the antennas are connected to the processor through the multiple amplitude-phase matched receivers and cable assemblies still used in older system architectures. The antennas and receivers are significantly more expensive as well. Add these factors to the complexity of the overall DF system design and the antenna array continues to be the major focus for new design challenges.
Computer and RF technology advances over the past 20 years have resulted in new tools and devices to create systems with a single receiver. Using one receiver in a system reduces the balancing errors, overcomes the complexities of amplitude-phase matched receivers and substantially reduces system cost. Hence, the system becomes more affordable.
There are various ways of designing a DF system with a single receiver, such as the RF combining subsystem approach. In this design, RF processing elements combine the outputs of the antenna elements and feed the combined RF output signal to a single receiver and DF processor.
An alternative approach uses RF sampling methods from an array of antennas. Here, an RF sampling (or commutating) switch samples each antenna and sends the combined samples to the receiver and DF processor. This sampling method simplifies the design of the antenna electronics and further reduces design complexity.
This approach is more affordable because of low-cost RF switching technology. In general, the array sampling techniques are also less expensive to implement than the RF combining methods. As the commutating switch samples the elements of the array, the commutation modulates the signal going to the receiver. The modulation arises from the phase differences between the antenna elements as the switch moves from one antenna to the next. The relative phase differences are related directly to the bearing angle.
Effectively, the known sampling process can be viewed from the perspective of the commutating switch electronically rotating the antenna and hence imposing a "Doppler spectral line" on the received signal. The spectral line is detected and processed to derive the bearing angle. This is the basis of pseudo-Doppler DF systems. Pseudo-Doppler DF systems are very good at determining bearing angles, however, they have three disadvantages. These problems have been addressed by U.S. patent application Ser. No. 07/998,391, entitled, "Method and Apparatus for Adaptively Determining the Bearing Angle of a Radio Frequency Signal", assigned and it is incorporated in its entirety.
Radio propagation conditions in the HF (High Frequency, 1-30 Mhz) portion of the spectrum often results in co-channel interference. In general, the interfering signals are not at the same transmission frequencies. But they are near each other in the spectrum. Consequently, when a receiver intercepts and detects the desired signal or signal of interest, it also receives and detects the interfering signal. Generally, the interfering signal is uncorrelated with the desired signal.
Direction Finding systems using small aperture antennas (antenna arrays) use amplitude, phase or doppler frequency methods to measure the bearing angle of a signal. When co-channel interference is present, all three parameters (amplitude, phase and doppler frequency) are corrupted. Correspondingly, since the bearing measurements are derived from these parameters, the bearing measurements are in error. The present invention addresses these problems.